The present invention relates to a method and apparatus for the casting of metal articles using external pressure.
In lost foam or evaporable foam casting, a pattern is produced from a polymeric foam material, such as polystyrene, and has a configuration identical to the metal article to be cast. A porous ceramic coating is applied to the outer surface of the pattern. One or more patterns are located within an outer mold or flask and a polymeric foam gating system connects each pattern to a sprue. The space between the patterns and the flask is then filled with a finely divided inert material, such as sand, and the sand also fills the internal cavities within the pattern.
In the casting process, the molten metal is fed into the sprue and the heat of the molten metal will act to decompose the polymeric foam material of the gating system, as well as the pattern, with the molten metal then occupying the void created by ablation of the foam material, with the decomposition products of the foam passing through the porous ceramic coating on the pattern and being trapped within the interstices of the sand. On solidification of the molten metal, the resulting cast article has a configuration identical to the polymeric foam pattern.
It is recognized that the application of external pressure to the molten metal before solidification of the metal is completed can aid in the interdendritic feeding of the casting and prevent both the precipitation of hydrogen porosity and the formation of microporosity. For example, U.S. Pat. Nos. 4,854,368, 5,014,764, 5,058,653, 5,088,544 and 5,161,595 describe processes of lost foam casting utilizing external pressure. The aforementioned patents state that the polymeric foam pattern is placed in an outer pressure vessel having a hinged lid and sand is fed into the pressure vessel surrounding the pattern, as well as the gating system. The molten metal is fed into the sprue and the heat of the molten metal will slowly ablate the polymeric foam gating system and the pattern. After the casting is completely filled, and before the molten metal has solidified, the lid on the pressure vessel is closed and pressure is applied to the molten metal, as well as to the upper surface of the sand.
However, it has been found that the method as described in the aforementioned patents has certain drawbacks. The large pressure gradient that occurs on the sudden application of pressure can cause metal penetration defects. The molten metal front is relatively unstable and the gaseous foam decomposition products tend to push back the metal front causing instability. Furthermore, the gas resulting from the foam decomposition often becomes trapped in the molten metal when pushing back, causing microporosity defects.
Another drawback of the methods described in the aforementioned patents is the appearance of xe2x80x9cfoldsxe2x80x9d. A xe2x80x9cfoldxe2x80x9d is a defect that occurs on filling of the mold when the products of the foam decomposition become trapped between the molten metal fronts. Such folds appear at the casting surface. The molten metal front pushes both gas and liquid decomposition products resulting from the foam pattern decomposition to the porous ceramic coating interface. The gaseous by-products of the foam pattern decomposition escape through the porous ceramic coating, and, under perfect conditions, it is contemplated that the heat from the molten metal will eventually evaporate the liquid by-products into gaseous form. However, when the liquid decomposition products become trapped between metal fronts, such liquid by-products never reach the coating. As the liquid fails to reach the coating, it is unable to escape through evaporation. The liquid becomes trapped, and results in the creation of folds in the casting surface. The folds are undesirable as they create weak sections in the surface and also can create porosity in the surface.
Further, the casting system as described in the above patents requires that each flask be a pressure vessel, and in commercial production this is a serious economic drawback. For example, when casting large engine blocks for internal combustion engines, the pressure vessel which serves as the flask must necessarily have substantial size and bulk. The cost of producing a pressure vessel of this size is quite substantial, and in commercial production where castings are made on a continuous basis, the overall cost of the quantity of pressure vessels required for production casting can make the system economically prohibitive.
The invention is directed to an improved method of casting utilizing external pressure and has particular application to lost foam casting. In accordance with a method of the invention, a generally cylindrical, metal flask or mold is placed within a pressure vessel having a removable lid. Located within the flask is one or more ceramic coated patterns formed of a polymeric foam material, such as polystyrene, and the patterns are connected through a gating system to a sprue located at the upper end of the flask.
A finely divided, unbonded, inert material, such as sand, surrounds the patterns and the gating in the flask and fills the internal cavities in the pattern. Located in the upper end of the pressure vessel is a pouring cup which communicates with the sprue, and the pouring cup has a relatively large volume, with the volume of the cup being at least 15% of the combined volume of the pattern and gating.
In the casting procedure, with the lid of the pressure vessel in the open or removed condition, a molten metal, which can take the form of an aluminum alloy, steel, or other alloy, is fed into the pouring cup. The heat of the molten metal will progressively decompose the polymeric foam material of the gating, with the gaseous products of decomposition passing through the porous ceramic coating on the foam and being trapped in the interstices of the sand. As the molten metal front progressively passes through the gating, it will come into contact with the pattern, and similarly, the foam material of the pattern will be decomposed by the heat of the molten metal, with the gaseous products of decomposition passing through the porous coating on the pattern and being trapped in the interstices of the sand.
In accordance with the invention, after the pouring cup is filled with the molten metal and before the molten metal front has decomposed the pattern, the lid on the pressure vessel is closed and sealed, and an external pressure, preferably in the amount of 5 to 50 atmospheres is applied to the molten metal in the cup, as well as to the upper surface of the sand in the flask. The pressure is maintained until the molten metal solidifies into the final cast article.
With the invention, the large volume pouring cup supplies the molten metal needs of the casting after the lid is closed, and thus pressure is applied before the molten metal filling of the casting is complete. This permits the early gentle application of pressure to avoid sand penetration defects that have occurred in prior casting methods. By maintaining pressure on the molten metal during filling, the molten metal front is more stable and the liquid and gaseous products of foam decomposition are less likely to push back the molten metal front and become trapped in the metal. Instead, the by-products of foam decomposition are pushed under pressure to the permeable coating on the pattern and can exit and subsequently condense in the sand grains.
When dealing with aluminum silicon alloys, the maintenance of pressure on the molten metal during filling stabilizes the molten metal front and decreases the presence of folds in the casting surface. If pressure is not maintained during filling, it is reasonable to expect the metal front to become unstable. If the metal front grows unstable, the instabilities appear as separate xe2x80x9cfingersxe2x80x9d in the molten metal front. This xe2x80x9cfingeringxe2x80x9d of the molten metal front has been observed through real-time x-ray viewing of the lost foam filling event and is documented by the U.S. Department of Energy Lost Foam Consortium under the direction of Dr. Charles Bates, Ph.D. http://www.eng.UAB.edu/mte/about/research/xray, (June, 2002). As the instability grows, the fingers grow and encounter their neighbors, entrapping liquid from the decomposition of the pattern between the fingers. When the fingers reach the porous ceramic coating, they trap the liquid within the metal, preventing the liquid from coming into contact with the coating, further preventing the liquid from evaporating, and ultimately creating folds in the casting surface.
With the invention, pressure is applied before filling is complete. The pressure stabilizes the metal front well inside the coating boundaries and prevents the xe2x80x9cfingeringxe2x80x9d effect as the metal front moves through the pattern. Thus, applying pressure before filling is complete attacks the fold defect problem by creating a more stable front. Furthermore, the application of pressure during filling facilitates the creation of polymeric decomposition gasses, creates a smaller gas gap between the molten metal front and the liquid decomposition products, and results in a quicker exit of gaseous decomposition products through the coating. Only this constant pressure application during filling, and not the application of pressure after filling, can assist in facilitating the elimination of such foam related defects.
A further advantage of the invention is that the pressure vessel, which is an expensive structure, is not used as the mold or flask, but instead an inexpensive lightweight flask is used inside of the pressure vessel. This is of particular advantage in the commercial production, particularly of large cast objects, such as engine blocks, where a pressure vessel must be of considerable size and bulk to house the cast engine block.
Further, the cylindrical shape of the flask enables vertical compaction of the sand to be used and this minimizes sand compaction faults.
Other features, objects and advantages will appear in the course following description.